1. Field of the Invention
The present invention relates to a magnetostrictive device which employs magnetostrictive bodies. The magnetostrictive device according to the present invention may be employed as a vibration sensor, an acceleration sensor, an actuator, a motor, a torque sensor or the like. Specific examples of application as an acceleration sensor include an automotive engine knock sensor and an automotive safety system acceleration sensor, whereas specific examples of application as a vibration sensor include an error state detection sensor in a rotary machine and the like.
2. Discussion of Background
Well known acceleration sensors, vibration sensors, torque sensors and the like in the prior art employ piezoelectric elements. A sensor that employs piezoelectric elements takes advantage of the phenomenon in which, when an external force caused by vibration or the like is applied to a piezoelectric element, an electric charge signal corresponding to the external force is generated from the piezoelectric element.
However, since a piezoelectric element has a high internal impedance, the electric charge signal is extremely sensitive to outside influences and tends to pick up noise. Consequently, in order to perform measurement with a high degree of precision, it is necessary to convert the electric charge signal to a voltage signal with a low impedance before the electric charge signal is input to a measuring device. This poses a problem in that an electronic circuit for impedance conversion and a power source are required, making the apparatus large and bulk.
Furthermore, when the ambient temperature exceeds approximately 120.degree. C., polarization degradation occurs in a piezoelectric element, which, in turn, degrades the piezoelectric characteristics and reduces the product life. In addition, degradation of the piezoelectric characteristics also occurs when high static electricity is accumulated in the piezoelectric element. Moreover, there is an added problem in that a piezoelectric element does not have a high degree of mechanical strength.
With a sensor employing a magnetostrictive body, such problems occurring in a sensor employing a piezoelectric element can be solved. In a sensor employing a magnetostrictive body, since, when an external force is applied to the magnetostrictive body, a voltage signal is generated in the coil which surrounds the magnetostrictive body, it is not necessary to use an impedance conversion circuit. In addition, it operates in a stable manner at high temperatures, and degradation of the characteristics due to high voltages such as static electricity does not occur.
The literature on the prior technology of vibration sensors employing a magnetostrictive body include U.S. Pat. No. 4,161,665. In U.S. Pat. No. 4,161,665, a vibration sensor used for the detection of engine knock is disclosed. This vibration sensor features a structure in which a cylindrical Fe--Ni magnetostrictive member with a coil wound around it is provided together with a magnetic circuit constituted with a yoke and a magnet, within a housing. The magnetic circuit applies a bias magnetic field to the magnetostrictive member. A blade spring is provided between the housing and the magnetostrictive member so that a pre-stress can be applied to the magnetostrictive member with this blade spring. When vibration is applied in the longitudinal direction relative to the magnetostrictive member, the magnetic permeability of the magnetostrictive member changes due to dimensional change in the magnetostrictive member in its longitudinal direction caused by the vibration. This, in turn, causes the magnetic flux which is interlinked with the coil to change, inducing a voltage in the coil.
However, U.S. Pat. No. 4,161,665 only discloses a sensor which detects vibration in one direction and does not disclose technology for detecting vibration in two or more directions.